The present invention relates to an organic EL display device capable of gray scale display by varying a duty ratio, a display device capable of binary display such as liquid crystal and FED, and to their drive method.
An organic EL display device of an active matrix type is a self luminescence display device characterized in high efficiency, high luminance and a wide viewing angle. Practical applications of such organic EL display devices are being developed. In order to realize gray scale display, an analog memory and a voltage-current conversion circuit are provided in each pixel circuit, and an organic EL element drive current is controlled by the voltage in the analog memory. However, there is a large variation in transistor characteristics so that a variation in emission luminance is large and display luminance is irregular, resulting in a difficulty of improving the image quality. In a digital display drive method, EL devices are controlled to take either an on-state or an off-state by using a pixel switch transistor.
This technique is detailed in JP-A-08-241048. Each pixel circuit has a digital memory made of one TFT and one capacitor, the on/off state of the organic EL devices are controlled by an output from the memory. This technique has considerably improved the luminance uniformity in the pixel on-state.
With the display device of this type, one frame period is divided into a plurality of sub-field periods, and a predetermined display period starts after scanning one frame to control the on/off state of each pixel. This operation is repeated to realize gray scale display of each pixel. If a large scale matrix is used, the wiring delay to be caused by wiring resistance and capacitance becomes considerably large, so that the necessary scan time for each sub-field prolongs and the display time becomes insufficient. In order to improve display luminance, it is necessary to use a large current operating point which provides a low light emission efficiency of EL, resulting in a possibility of an increase in the panel power consumption. If the panel is made large, the wiring delay increases considerably, and the frame period prolongs. In this case, flicker and the like occur and the performance of moving image display lowers.
According to the conventional technique described above, organic EL devices of a pixel circuit are driven to have a binary state in order to remove a variation in display luminance. In order to obtain gray scale drive, one frame period is divided into a plurality of sub-field periods. All pixels are scanned in each sub-field period to write binary display data corresponding each gray scale level bit, and during the display period, each pixel is turned on at a predetermined luminance and for a predetermined time.
If the number of gray scale levels is increased to improve the image quality, the number of sub-fields increases and the scan frequency becomes high. For example, if a display device having 640xc3x97480 pixels is driven at an 8-bit gray scale level, a frame frequency of 60 Hz, a horizontal blanking period of 20%, and a display period of a half of one sub-field period, then the scan frequency is 60xc3x97480xc3x971.2xc3x978xc3x972=552 kHz and one horizontal scan period is 1.8 xcexcsec. As compared to a scan frequency of the analog drive type is 34.6 kHz, an operation speed 16 times faster is required.
Therefore, as compared to the analog drive type pixel, a wiring delay to be caused by wiring resistance and capacitance in a pixel circuit is required to be reduced further by considerably lowering the wiring resistance and capacitance. It is therefore necessary to thicken wiring lines and interlayer insulating films, which results in a low manufacture yield, a complicated process and an increased cost. If high precision and the increased number of gray scale levels are to be realized for improving the image quality or if the display device is made large, the scan frequency becomes higher so that a high image quality and a large screen display device are difficult. An increase in the scan frequency results in an increase in a circuit power consumption and a necessity of using a high speed signal processing circuit, so that a heat generation amount of the panel increases.
In consideration of the above-described problems associated with the conventional technique, it is an object of the present invention to provide a display device and its driving method capable of gray scale display at a high precision and reduction of a power loss.
In order to achieve the above object of the invention, an on/off of each pixel is controlled in order to make display luminance of pixels uniform, and in order to effectively use the display period, gray scale control is realized by controlling the ratio of turn-on time to the frame period of each pixel not by using sub-fields of conventional technique.
To this end, according to a first aspect of the invention, each pixel is provided with a signal sampling circuit, a time constant circuit or constant current circuit and a voltage comparator circuit. The signal sampling circuit is made of a transistor and a capacitor, and samples an analog signal voltage corresponding to display luminance. The time constant circuit or constant current circuit changes the sampled signal voltage with time. The voltage comparator circuit compares a continuously changing sampled voltage with a comparison reference voltage to judge an amplitude state of both the voltages.
According to a second aspect of the invention, in addition to the circuits described above, each pixel is provided with a reference voltage sampling circuit, a time constant circuit or constant current circuit and a voltage comparator circuit. The reference voltage sampling circuit samples a reference voltage. The time constant circuit or constant current circuit changes the reference voltage with time. The voltage comparator circuit compares a continuously changing sampled reference voltage with the sampled signal voltage to judge which one of both the voltages is higher.
According to a third aspect of the invention, each pixel is provided with a signal sampling circuit and a reference voltage sampling circuit. The signal sampling circuit is made of a transistor and a capacitor, and samples an analog signal voltage corresponding to display luminance. The reference voltage sampling circuit samples a reference voltage. A reference voltage capacitor sampled the reference voltage is coupled between the reference voltage and a voltage comparator circuit so that the voltage comparator circuit compares a difference voltage from the sampled reference voltage with the sampled signal voltage.
With the first to third aspects of the invention, driving the pixel circuit is controlled to control the ratio of a turn-on time.
In the first aspect of the invention, a signal voltage is sampled at a pixel selected by a scan line under line-at-a-time scan. The signal voltage at the end of the selection period sampled in the capacitor lowers with time in the time constant circuit. The voltage comparator circuit compares the sampled voltage with the reference voltage. A control voltage at the output terminal of the voltage comparator circuit changes when the amplitude state of both the voltages is inverted. The control voltage controls the conductive/non-conductive state of the main circuit of an EL driver circuit. Only while the main circuit is conductive, the organic EL devices of the pixel circuit are turned on.
In the second aspect of the invention, a signal voltage and a reference voltage are sampled at a pixel selected by a scan line under line-at-a-time scan. The reference voltage at the end of the selection period sampled in the capacitor lowers with time in the time constant circuit. The voltage comparator circuit compares the signal voltage with the reference voltage. A control voltage at the output terminal of the voltage comparator circuit changes when the amplitude state of both the voltages is inverted. Specifically, when the reference voltage is lower than the signal voltage, the comparator output is inverted. The control voltage controls the conductive/non-conductive state of the main circuit of an EL driver circuit. Only while the main circuit is conductive, the organic EL devices of the pixel circuit are turned on.
In the third aspect of the invention, a signal voltage and a reference voltage are sampled at a pixel selected by a scan line under line-at-a-time scan. The reference voltage at the end of the selection period sampled in the capacitor is inserted between the reference voltage wiring line and the input terminal of the voltage comparator circuit. In this case, this connection inverts the polarity of the voltage relative to the voltage comparator circuit. Therefore, a relative reference voltage corresponding to the reference voltage input terminal voltage of the voltage comparator circuit immediately after the selection period is generally 0. Thereafter, this voltage at the input terminal changes relatively in accordance with a voltage change on the reference wiring line. The voltage comparator circuit compares the signal voltage with the relative reference voltage. A control voltage at the output terminal of the voltage comparator circuit changes when the sign of subtraction between both the voltages is inverted. The main circuit of an EL driver circuit is made conductrive and non-conductive by the control voltage. Only while the main circuit is conductive, the organic EL devices of the pixel circuit are turned on.
According to the present invention, a pixel circuit uses organic EL devices and has a built-in comparator circuit. Accordingly, the light emission time of each pixel can be controlled so that even if the characteristics of transistors constituting the pixel circuit vary, a variation in luminance is small and a display device capable of gray scale display at a high precision can be provided. Since a pixel power consumption depends on the on/off state of OLED, the drain power loss of the transistor can be reduced and a display device capable of high efficiency and low power consumption can be realized.
In the pixel circuit with the comparator circuit, a time constant circuit is used so that the circuit structure can be made simple. The number of components is therefore small and a display device with a high precision can be provided. In the structure that an external triangular wave is applied to compare it with a sampled voltage in the pixel and control the light emission time, the light emission time can be controlled at a high precision and this structure is effective for multi-level gray scale.